A planar Fourier capture array (PFCA) is an image sensor that does not require a lens, mirror, or moving parts. As a consequence, cameras that employ PFCAs to acquire image data can be made extraordinarily small and inexpensive. PFCAs include angle-sensitive pixels whose sensitivity to light is a sinusoidal function of incident angle within the imager's field of view. The measurement from one photodiode from a PFCA can be interpreted as a measure of one component of the two-dimensional (2D) Fourier transform of a far-away scene. Each pixel has physical characteristics that make is sensitive to a distinct component of the 2D Fourier transform of the far-away scene. Taken together, these components relate full Fourier information representative of the scene. Some applications may use the Fourier components directly, or images of the scene can be computationally reconstructed.
PFCAs exploit a near-field diffraction effect named for Henry Fox Talbot (the “Talbot effect”). Briefly, a plane wave incident upon a periodic diffraction grating produces a repeating image of the grating at regular distances away from the grating plane. A second grating, or “analyzer,” beneath the first grating passes or blocks the image depending on the incident angle. The resultant pattern is then captured by a conventional photodetector array. Finally, the subject of the image is resolved computationally from the captured pattern.
The spacing between the grating layers, and between the grating layers and the photodetector array, can be very difficult to manufacture with sufficient precision to ensure that the analyzer layer and the photodetector array fall precisely at the regular distances that accurately reproduce a Talbot image. In standard CMOS processes, for example, interlayer thicknesses can vary by 20%. Also problematic, Talbot spacing is a strong function of wavelength, making it exceedingly difficult to produce sharp Talbot images over some wavelength bands of interest (e.g., the visible light spectrum).